1. Field of the Invention
The present invention relates to cold, non-equilibrium glow discharge plasmas, particularly those at or about atmospheric pressure.
2. Background Literature and Patent References
Industrial Plasma Engineering, Volume 1xe2x80x94Principles. Institute of Physics Press, Bristol, UK ISBN 0-7503-0318-2, provides comprehensive and thorough information on several aspects of plasma technology. The book is incorporated herein by reference.
Informative patents that also provide background information are the following: U.S. Pat. Nos. 5,387,842, 5,669,583, 5,414,324, 5,403,453, 5,456,972, which are incorporated herein by reference.
3. Description of the Related Art
It is well known to expose a workpiece to a glow discharge plasma to change one or more properties of the workpiece. For example, it is known to generate a glow discharge plasma between two parallel-plate electrodes, and to pass a continuous web or film between the electrodes and therefore through the discharge plasma to change, e.g., the wettability of the web or film. In such situations, the workpiece is exposed to the discharge plasma in the same region in which the plasma is generated. In some circumstances, however, this so-called direct exposure to the plasma may result in a high-power flux of active species which may heat and damage the workpiece surface, but which does not contribute to the desired effects. In addition, direct exposure to the plasma may cause the workpiece to be bombarded by all active species, including electrons, ions, ultraviolet radiation, ozone, and molecular fragments, some of which may be damaging or unhelpful to achieving the desired result of exposure to a particular active species.
Moreover, direct exposure implies subjecting the workpiece to the same electromagnetic fields that are used to generate the plasma, which again may be detrimental to the workpiece. For example, generating atmospheric plasmas may require electric fields of at least several kilovolts per centimeter in order to maintain a plasma at one atmosphere (e.g., a minimum of 8.5 kilovolts per centimeter in air for a one-atmosphere uniform glow discharge (OAUGD) plasma). When the plasma-generation region is a space between two parallel plates, the immersion of a reasonably large workpiece in the plasma between such parallel plates can require voltages so high that they become a serious occupational safety hazard to those using the equipment (e.g., due to sparking, X-ray production, and radio frequency (RF) interference). Such hazards may make such a unit impractical, for example, for use as sterilization units for medical or dental offices.
The present invention is directed to techniques for remotely exposing workpieces to OAUGD plasmas. According to these techniques, some of the plasma active species are propelled from the region in which the plasma is generated into a remote region where the workpiece is positioned for exposure to one or more constituents of the plasma without necessarily exposing the workpiece to all of the plasma constituents or to the strong electric fields within the plasma-generation region. According to the present invention, the active species are convected by any suitable means from the plasma-generation region to the remote-exposure region without necessarily requiring an external force to be applied to move the plasma.
Plasmas like the OAUGD plasma can benefit by decoupling the plasma-generation region from the remote-exposure region in which a workpiece is exposed to the active species that do the work of plasma processing. Such a remote-exposure reactor not only enables the generation of plasma-active species at one atmosphere without expensive vacuum systems, but it also enables the convection of active species, e.g., for sterilization or increasing the surface energy of materials, away from the plasma-generation region into a remote chamber where objects of any size, shape, or porosity can be treated without respect to the plasma operating conditions, including the required operating voltage. Active species can include ultraviolet or visible photons; charged particles, including electrons, ions and free radicals; and highly reactive neutral species, such as reactive atoms (oxygen, fluorine, ozone, nitrogen oxides, etc.), excited atomic states, and reactive molecular fragments, such as monomers.
In the present invention the active species which produce the desired effect are convected by the neutral gas flow away from the plasma generating regions, where the surfaces may experience a flux of detrimental active species. Thus, an effect of the invention is the convection of desired active species such as oxidizing species, like atomic oxygen, ozone, nitrogen oxides, away from the region of high heat fluxes, ultraviolet radiation, and strong electric fields where the plasma is generated.
The ability to decouple the generation of the plasma from the exposure site of the workpiece will greatly enhance the utility of the OAUGD plasma technology and open up many industrial uses that would otherwise either be difficult to implement, or difficult to sell to potential customers.
In one embodiment, the present invention is a method for treating a workpiece with one or more active species from a plasma, comprising the steps of (a) applying power to generate an electric field within a plasma-generation region to generate a volumetric plasma; (b) convecting the one or more active species away from the plasma-generation region as a result; by one of the selected means, such as a blower, and (c) subjecting the workpiece to the one or more active species, wherein the workpiece is located outside of the plasma-generation region such that the workpiece is not directly subjected to the plasma or to the electric field within the plasma-generation region.
In an alternative embodiment, the present invention is a remote-exposure reactor, comprising (a) a plasma generator defining a plasma-generation region and having one or more plasma panels adapted to be configured to a power supply to generate a plasma within the plasma-generation region, wherein one or more active species of the plasma are convected away from the plasma-generation region as a result of the means of propelling the plasma; and (b) means for subjecting a workpiece located outside of the plasma-generation region to the one or more active species such that the workpiece is not directly subjected to the plasma or to the electric field within the plasma-generation region.